A wide range of electrochemical methods, including amperometric and potentiometric techniques, presently use liquid filled reference electrodes.
The purpose of a reference electrode in potentiometry is to provide a steady potential against which to measure the working electrode half-cell (for example, an ion-selective electrode, redox potential electrode or enzyme electrode). In amperometric techniques, two electrode cell configurations are widely used (working and counter electrode half-cells) but, particularly in more accurate work, three electrode cell configurations are used, where the reference electrode is used to provide a sample reference potential.
The most common types are calomel (using mercury/mercury chloride couple) and silver/silver chloride reference electrodes. Thus the metal electrode (Ag or Hg) is bathed in a halide salt solution, which is connected to the sample through some form of liquid junction, for example a porous ceramic frit.
The potential of the metal/metal halide couple within the device is used as the reference potential, on which is superimposed the potential of the liquid junction and any other potential exerted, for example, by the sample at the liquid junction. In order to maintain a steady potential, these reference electrodes usually contain a concentrated halide solution (for example, saturated potassium chloride), to minimise the variation in reference potential on diffusion of sample through the liquid junction into the inner halide solution. Further, the device is normally mounted so that the level of the inner filling solution is higher than that of the sample, so that the inner filling solution slowly leaks from the liquid junction, helping to maintain the inner filling solution and liquid junction with steady levels of electrolyte (i.e. the inner filling solution) and potential.
Particularly in the case of ion-selective electrodes (ISEs), rather than using a conventional reference electrode, another ion-selective electrode can be used as a reference against which the potential of the working ISE half-cell is measured. This approach requires that the activity (concentration) of the ion to which the "reference ISE" is selective, remains constant or changes in a known manner. Any other species in the sample which interferes with the "reference ISE" must also meet the same requirement. In most practical applications with complex and dynamically changing samples, this can be difficult or impossible to arrange. The approach is accordingly not widely used.
The main problems and disadvantages of conventional liquid-filled reference electrodes, with particular regard to ISE's, are believed to be:
i) comparatively large size (the liquid junction connection cannot, for example, be made smaller or of lower leakage without seriously affecting its performance);
ii) incompatibility with solid state working ISE manufacture processes and application configurations;
iii) leakage of strong/concentrated internal electrolyte changing the sample (for example, KCl leakage in K.sup.+ determination);
iv) fouling or blockage of porous ceramic frit or other liquid junction;
v) replacement or topping-up of internal electrolyte solutions and other servicing required relatively frequently;
vi) possible variation in liquid junction potential with changes in the sample electrolyte concentration/ionic strength;
vii) poor impedance matching of the reference electrode (few Kilohm) to the working ISE (often Megohm to Gigohm), encouraging series mode electrical interferences and drift.
Attempts have been made to produce solid state or miniaturised versions of the above reference electrode. Essentially, the Ag/AgCl electrodes are overcoated with a hydrophilic polymer gel containing the internal electrolyte (for example, KCl) and some form of porous membrane or frit is then introduced. The main additional problems and limitations of these approaches are:
a) the reservoir of internal electrolyte is much smaller and its concentration therefore more easily changed by the sample;
b) it is most often difficult or impossible to ensure that internal electrolyte leaks from the device at a sufficient rate to maintain a steady potential at the liquid junction;
c) there is often a rapid drying out of the device which changes electrolyte concentrations, leading to salting-up of the junction and ultimately causing device failure.
For these reasons, and especially in extended storage or use, such solid state devices will be likely to drift or fail. Particularly in the case of disposable "one shot" tests, some have partially circumvented these problems by adjusting the internal electrolyte solution to be the same as the sample (usually a lower concentration) and placing the same electrolyte solution in a gel layer mounted outside the liquid junction. The latter layer is then removed immediately prior to contact with the sample. Electrolyte equilibration between the sample and reference electrode thereby becomes less problematical. The technique is not however capable of wide application.